2148
†To whom correspondence should be addressed.
E-mail: [email protected]
Korean J. Chem. Eng., 28(11), 2148-2155 (2011)DOI: 10.1007/s11814-011-0090-9
INVITED REVIEW PAPER
Application of rare earth as mordant for the dyeing of ramie fabrics with natural dyes
Guang Hong Zheng*,†, Hong Bin Fu*, and Guang Ping Liu**
*Dyeing and Chemistry Department of Chengdu Textile College, Chengdu 61006, China**Chongqing 3533 Printing Dyeing and Clothing Co., Ltd., Chongqing, P. R. China
(Received 14 November 2010 • accepted 4 April 2011)
Abstract−Selecting appropriate metallic compounds as mordants is essential for dyeing with natural dyes. This paper
presents the application of rare earth compounds as mordant for the dyeing of ramie fabrics with four kinds of natural
dyes. The influences of pre-mordanting, simultaneous mordanting, and post-mordanting on the dyeing effect were ex-
plored. The post-mordanting was proved to give rise to the highest dye uptake. The effects of dyeing conditions including
dyeing temperature and time, dyeing bath pH and the concentration of rare earth on the dye uptake were investigated
systematically. The fabrics dyed with natural dyes in presence of rare earth as mordant exhibited high color shade stability
in the baths with pH varying from being acidic to neutral and alkaline. Employing rare earth as mordant apparently
raised the color fastness to washing, rubbing and light of the ramie fabrics dyed with the natural extracts. In comparison
with the commonly used metallic salts, using rare earth chlorides as mordants can greatly reduce the ionic concentration
employed in natural dyeing. This study proved that rare earth would be a kind or promising environmentally friendly
mordant in natural dyeing.
Key words: Natural Dye, Mordant, Rare Earth, Color Fastness
INTRODUCTION
The colorants used in commercial textile dyeing are almost exclu-
sively synthetic.
However, synthetic dyes not only are harmful to health and but
also result in a great deal of environmental pollution [1-6]. As a result,
natural dyeing in textiles is now attracting more and more attention
from both academia and industry due to its environmentally friendly
attributes [1,2,4-16]. Natural dyes can exhibit better biodegradabil-
ity and generally have a higher compatibility with the environment
[3,4,17-19].
Dyeing fabrics with natural dyes often leads to problems such as
lower color fastness to washing or light of the dyed textiles [12,14,
20-22]. Most attempts for overcoming these problems involved the
use of metallic salts (e.g., aluminum potassium sulfate, potassium
dichromate, stannous chloride, ferrous sulfate and copper sulfate)
as mordants [3,5,6,9,11-13,23-26]. The metal ions can act as elec-
tron donor to form coordination bonds with the dye molecules. The
wastewater-containing heavy metal ions from these mordants have
significant impact on the environment and public health [25]. The
content of heavy-metal ions in textile is limited. Therefore, select-
ing new mordant to replace traditional heavy-metal ions has been
an important part in the development of natural dyeing of textiles.
Rare earths are ecofriendly and compatible with the environment.
In general, the coordination number of rare earth can be as large as
8, 9 and even 12 [27,28]. Additionally, the rare earth elements have
large atomic radius, which enables a good number of ligands sur-
rounding them [27]. Hence the rare earth ions exhibit high capability
for forming coordination compounds with natural dye molecules.
As the central ions, the rare earth ions can form coordination bonds
with the amino, hydroxyl or carboxyl groups, i.e., the ligands, of
the natural dye molecules. When forming coordination compounds
with natural dye molecules, the rare earth ions tend to exhibit electro-
lyte-like effect and thus quickly lower the Zeta electric potential on
the surface of fibers. Therefore, they are easily absorbed on the sur-
face of fiber by static electric force. Using rare earth products as
mordants can promote the formation of coordination compounds of
natural dyes, rare earth and fibers and thus enhance the color fastness
of the fabrics dyed with natural extracts.
Our aim here is to study the application of rare earth products as
mordant for natural dyeing. The lanthanum-rich rare earth chloride
was used as the mordant in the natural dyeing of ramie fabrics. The
dyeing conditions including mordanting method, dyeing temperature
and time, pH value of dyeing bath and concentration of mordant
on the dyeing effect were investigated systematically. The color shade
stability of the dyed ramie fabrics with rare earth as mordant was
examined in the solutions having different pH values. The effects
of using rare earth as mordant on the color fastness to washing, rub-
bing and light of the ramie fabrics dyed with natural dyes were studied.
As compared with conventional metallic mordant ferrous sulfate
(FeSO4) and potassium dichromate (K2Cr2O7).
EXPERIMENTAL
1.Materials
Ramie plain fabrics (21×21) were used in this study. The fabrics
were treated with a solution containing 5 g/l non-ionic detergent (Hos-
tapal CV, Clariant) at 50 oC for 30 minutes prior to using. Then the
fabrics were thoroughly washed with water and air dried at room
temperature for usage. Caesalpinia sappan, Rhizoma coptidis, Gared-
nia and Areca catechu were provided by the Chongqing 3533 Print-
Application of rare earth as mordant for the dyeing of ramie fabrics with natural dyes 2149
Korean J. Chem. Eng.(Vol. 28, No. 11)
ing Dyeing and Clothing Co. Ltd. The lanthanum-rich rare earth
chloride (RECl3·6H2O) (REO≥50%, La2O3/REO=43%, CeO2/REO
=40%, Pr6O11/REO=5.0%, Nd2O3/REO=18%, Sm2O3/REO=1.5%,
Eu2O3/REO=0.2%, Tb4O7/REO=0.05%, Non-RE Impurities/REO
=2.69%) and neodymium chloride (NdCl3·6H2O) (REO≥50%, Nd2O3/
REO≥99%, Non-RE Impurities/REO≤1%) were purchased from the
Inner Mongolia Baotou Steel Rare-earth (Group) Hi-Tech Co., Ltd.
Ferrous sulfate (FeSO4·7H2O) (Analytical Reagent grade, Chengdu
Best Reagent Co., Ltd.) and potassium dichromate (K2Cr2O7) (Ana-
lytical Reagent grade, Chengdu Best Reagent Co., Ltd.) were used
as received. Sodium hydroxide, acetic acid and lemon acid of chem-
ical pure grade were purchased as from Sinopharm Chemical Reagent
Co., Ltd.
2. Extraction of Natural Dyes
At first, 150 g of a selected plant was precisely weighed with the
analytical balance (TE1502S, Sartorius Co., Germany). After being
mashed it was put into a big beaker containing 800 ml of water and
kept for 24 hours. Afterwards, the mixture was heated to boiling
state where it was maintained till up to only 100 ml liquid being
left via evaporation. Then the dissolving and evaporating processes
were repeated twice. After filtration, the collected colorant liquid was
distilled and condensed to 50 ml. The concentrated solution con-
taining the plant extracts was the crude natural dye liquid. The ultravi-
olet-visible (UV-vis) spectra of the aqueous solutions containing the
natural dyes were obtained from U-3310 UV-vis spectrophotome-
ter. As indicated in Table 1, the maximum absorption wave length
values of the solutions containing natural dyes range from 410 nm
to 460 nm, which are dependent on the different component com-
positions of the natural dyes. According to the maximum absorption
wave length values, it is judged that the major colorant ingredients
of the different natural extracts are brazilin of caesalpinia sappan,
berberine of rhizoma coptidis, crocin and crocetin of gardenia, and
catechin of areca catechu. Fig. 1 illustrates the chemical structure
of the major colorant ingredients of the different natural extracts.
3. Dyeing
In the dyeing process, a concentrated plant extract liquid was diluted
by 10 times and the resultant solution was used as dyeing bath. The
Table 1. The maximum absorption wave lengths of the solutionscontaining natural dyes
Caesalpinia
sappan
Rhizoma
coptidisGardenia
Areca
catechu
Maximum
absorption wave
length (nm)
412 452 458 458
Fig. 1. Chemical structure of the colorant ingredients of different natural extracts.
2150 G. H. Zheng et al.
November, 2011
aqueous extracts were used as a direct dye or with the addition of a
mordant. The direct dyeing was carried out by shaking the ramie
fabrics with a natural dye solution in a conical flask at 90 oC in a
thermostat shaker bath operated at 100 strokes/min. A material to
liquid ratio of 1 : 25 was used in the dyeing experiment. The ramie
fabric was then rapidly withdrawn after particular immersion times.
After dyeing, the ramie fabric was rinsed with deionized water to
remove the unfixed dye and then air dried. The dyeing effect of sim-
ultaneous mordanting, pre-mordanting and post-mordanting were
compared. Fig. 2 demonstrates the three mordanting methods at
the dyeing temperature of 90 oC.
In the simultaneous mordanting process, the fabrics were immersed
in a dyeing bath containing mordant, lemon acid and natural dye at
30 oC. They the dyeing bath was heated to 90oC where it was main-
tained for a particular time (20-80 minutes). The fabrics were then
rinsed with clean water at ambient temperature followed by being
squeezed and dried. In the pre-mordanting method, the wet fabrics
were first immersed in a solution of the mordant and heated to 70 oC
where they were kept for 30 minutes. After being added with the
natural dyes the bath was heated to 90 oC where the fabrics were
treated for a particular time (20-80 minutes). Then the dyed sam-
ples were rinsed with clean water and dried via the similar pro-
cesses mentioned above. In the post-mordanting method, the ramie
fabrics were first dyed in an aqueous solution containing a natural
dye at 90 oC for 30 minutes followed by being cooled to 70 oC where
the mordant was added. Then the fabrics were kept at 70 oC for an-
other 30 minutes. Subsequently, the dyeing bath was heated to 90 oC
again where the fabrics were dyed for a time in the range of 20-
80 minutes. At last, the fabrics were rinsed and dried. The effects
of dyeing temperature and time on the dyeing effect of ramie fabrics
with the natural dyes were studied.
4. Measurements
The dye uptake in the dyeing of ramie fabrics was obtained through
the measurement of the light absorbances at the wavelength of maxi-
mum absorption, of the dye bath before and after dyeing with a U-
3310 ultraviolet-visible (UV-vis) spectrophotometer. The dye uptake
was calculated with the following equation:
where A0 and At refer to the absorbances of the dye solutions at the
beginning and the end of dyeing, respectively.
The CIELAB colorimetric values including ∆E, L*, a*, b*, C*,
and the color strength K/S of the dyed fabrics were measured by
DatacolorSF600 Computer Color Matching System (Data Color
International) using illuminant D65 and 10o standard observer. K/S
was calculated from the reflectance values using the Kubelka-Munk
equation as follows:
Where R represents the reflectance of the dyed fabric and K/S stands
for the ratio of the absorption coefficient (K) to scattering coeffi-
cient (S). The higher K/S value the greater the color strength.
The color fastness to washing of the ramie fabrics was deter-
mined according to the standard ISO 105-C03. The measurement
was carried out with both sample and standard ramie fabrics that
were sewn together and tested under the same conditions. The sewn
fabric was washed at 42 oC for 30 minutes in a standard soap solu-
tion with a material to liquid ratio of 1 : 50. Both fabrics were then
separated and rinsed and dried. The color fastness to washing levels,
observed against a grey scale, were classified as numbers ranging
from 1 and 5, which corresponds to poor to excellent fastness, re-
spectively. Color fastness to dry and wet rubbing of the dyed fabrics
was tested according to ISO 105-X12 method. Color fastness to light
was tested according to ISO 105-BO2 method. To examine the in-
fluence of pH change on the color shade, the fabrics dyed in pres-
ence of rare earth as mordant were immersed in the water baths with
different pH from 3, 7 to 11 at ambient temperature for 30 minutes.
Then the wet fabrics were rinsed and dried for further measurement.
RESULTS AND DISCUSSION
1. Influences of Mordanting Method
The effect of different mordant dyeing methods is shown in Table
2. It was observed that the post-mordanting gave rise to the highest
color strength and dye uptake in comparison with simultaneous mor-
danting and pre-mordanting. In simultaneous mordanting, the colo-
rant molecules and rare earth ions could form insoluble coordination
Dye uptake =
A0 − At
A0
---------------- 100%×
K/S =
1− R( )2
2R-----------------
Fig. 2. Different mordanting methods in the natural dyeing.
Table 2. Influences of different mordanting methods
Caesalpinia sappan Rhizoma coptidis Gardenia Areca catechu
K/S Dye uptake K/S Dye uptake K/S Dye uptake K/S Dye uptake
Simultaneous mordanting 5.25 47.8 6.01 54.6 3.88 39.8 4.98 42.3
Pre-mordanting 5.86 52.2 6.58 60.1 4.23 43.5 5.45 49.8
Post-mordanting 6.12 58.0 7.29 64.1 5.07 48.4 5.96 54.6
Application of rare earth as mordant for the dyeing of ramie fabrics with natural dyes 2151
Korean J. Chem. Eng.(Vol. 28, No. 11)
compounds and thus precipitated from dyeing bath, resulting in the
decrease of effective dye uptake and K/S value, i.e., color strength.
In the case of pre-mordanting, the mordant ions absorbed on the
fibers could desorb from the fibers and form insoluble coordination
compounds with natural dye molecules. Likewise the pre-mordant-
ing resulted in lower color strength. Therefore, the post-mordant-
ing technique was employed in this study.
2. The Optimal Conditions for Dyeing
2-1. Influences of Temperature and Time on Dye Uptake
Fig. 3 presents the influence of dyeing temperature and time on
dye uptake. When the dyeing temperature was 40 oC, the dye uptakes
of the four kinds of natural dyes increased almost monotonically as
the dyeing time rose from 10 to 80 minutes. When the dyeing tem-
perature was 70 oC the dye uptakes of the natural dyes increased
rapidly at the beginning. With the extension of dyeing time the in-
creasing tendency of dye uptake decreased gradually. The dye up-
takes of the nature dyes were apparently enhanced as the dyeing
temperature increased from 70 oC to 90 oC. The increase of dye up-
take arises from both the increasing solubility of natural dyes and
the expansion of ramie fibers with the increasing temperature. In con-
trast, the dye uptake of the nature colorants was slightly decreased
when the dyeing temperature increased from 90 oC to 105 oC. This
is caused by the different temperature dependence of absorption
and diffusion of dye molecules. Dyeing takes place in three steps:
absorption on the surface of fibers, diffusion to the interior of fibers
and desorption from fibers into the dye bath [2]. On the one hand,
a lower dyeing temperature is favorable to absorption of the dye
molecules on fibers, but makes their diffusion inside fibers difficult.
On the other hand, a high temperature leads to expansion of fibers
and easy diffusion of natural dyes in fibers accordingly. However a
high temperature can also intensify molecular movement resulting
in desorption of more dye molecules from fibers. In a word, either
excessively low or excessively high temperatures cannot give a de-
sirable dyeing effect. The results show that 90 oC is a suitable tem-
perature for dyeing ramie fabrics with the four kinds of natural dyes.
As indicated by Fig. 3, the dye uptake rose gradually with the in-
crease of dyeing time for all the four kinds of natural dyes. When
the dyeing time increased to a certain time the dye uptake of a natural
dye tended to a constant value, indicating the dye uptake equilib-
rium was attained [2]. It takes 70, 60, 70 and 50 minutes for the ex-
tracts of caesalpinia sappan, rhizoma coptidis, gardenia and areca
catechu, respectively, to attain the dye uptake equilibrium, which
relies on the molecular size and polarity of the four kinds of natural
dyes.
2-2. Influence of pH Value on Dye-uptake
Fig. 4 shows the influence of pH of dyeing bath on the dye up-
take in the dyeing with the natural extracts. The figure indicates that
the pH change could significantly influence the dye uptake in the
dyeing with caesalpinia sappan and areca catechu. When pH value
was about 2 the dye uptake of the two kinds of colorants was fairly
low. With the increase of pH, the dye uptakes of them rose gradu-
ally. When pH value increased the maximum dye uptakes of the
extracts of caesalpinia sappan and areca catechu were attained at
pH=7 and pH=8, respectively. Afterwards, their dye uptake de-
creased with the further increase of pH value. The effect of pH on dye
uptake can be attributed to the interaction between the natural dyes
Fig. 3. Influences of dyeing temperature and time on dye uptake: (a) Caesalpinia sappan L; (b) Rhizoma coptidis; (c) Gardenia; (d) Arecacatechu. In each plot the symbols refer to different dyeing temperatures. (■ ) 40 oC; (●) 70 oC; (▲ ) 90 oC; (★) 105 oC.
2152 G. H. Zheng et al.
November, 2011
and the ramie fabric. A certain number of OH and COOH groups
exist in the cellulose structure. Brazilin and catechin contain polyphe-
nols in their molecules as illustrated in Fig. 1. Increase of pH could
promote the ionization of phenolic OH groups resulting in the in-
crease of solubility of the natural dyes in dyeing bath. Hence the
dye uptakes of the two natural dyes rose with the increase of pH
value. At higher pH, the hydroxyl-ion content increased, and phe-
nolic OH groups in the natural dye molecules and COOH groups
in the cellulose fibres were transformed to phenolic oxygen anions
and carboxyl anions, respectively. Phenolic oxygen anions had a
repulsive interaction with OH groups and carboxyl anions, which
prevented the natural dye molecules from being absorbed onto
ramie fibers and thus reduced the dye uptakes of the two natural
dyes [2]. As one of the major colorant ingredients of gardenia, croce-
tin contains COOH groups in its molecules. As indicated by Fig. 4,
with the increase of pH the dye uptake of the natural extract of gar-
denia increased, resulting from the increasing solubility of the natu-
ral dyes in dye bath due to the ionization of COOH groups in crocetin.
Likewise, the dye uptakes of the natural extract of gardenia decreased
when pH was above 7 because of the increasing repulsion between
the carboxyl anions on natural dye molecules and the carboxyl anions
and OH groups on ramie fibers. In comparison with those of caesal-
pinia sappan and areca catechu, the extract of gardenia exhibited
less dye uptake variation with the change of pH because of the rela-
tively low content of COOH groups in the molecules of its major
colorant ingredients. With the increase of pH the dye uptake of the
extract rose gradually as shown in Fig. 4. As the major colorant in-
gredient of rhizoma coptidis, berberine is a water soluble dye con-
taining cationic quaternary ammonium salt [9], it would interact
ionically with the carboxyl groups of cellulous fibers at higher pH
via ion exchange reaction. The number of available anionic sites
on ramie fibers in alkaline conditions is relatively larger than that in
acidic conditions, and thus the dye uptake of berberine was increased
with the dye bath pH.
2-3. Influence of the Amount of Rare Earth on Dye Uptake
The electron configuration of the outer shells of rare earth ions
enables them to form complex compounds with the natural dyes
and ramie fibers. The influence of mordant amount on the dye uptake
is shown in Fig. 5. It can be seen that the dye uptake increased with
the increase of the concentration of rare earth. However, when the
concentration of rare earth increased to a certain level the dye up-
take tended to a constant value because the coordination among
natural dye, rare earth and fiber reached to saturation state. The dye
uptake of the gardenia extract was the lowest among all the four
kinds of natural dyes. This should be due to the low affinity of the
crocin and crocetin components in gardenia for ramie fibers. In con-
trast, the dye uptake of the extract of rhizoma coptidis is the highest,
which can be ascribed to the ionic interaction between the cationic
dyes and carboxyl anions in ramie fibers. The influence of rare earth
concentration on dye uptake of the different natural colorants is deter-
mined by their complex formation from rare earth ions and natural
dye molecules.
3. Influence of pH on Color Shade of the Dyed Fabrics
It was found that the fabrics dyed with the natural dyes in pres-
ence of rare earth as mordant could resist the pH impact and thus
change less color shade in washing. To quantify the influence of pH
Fig. 4. The influence of pH value on dye-uptake. (■) Caesalpiniasappan L.; (▲ ) Rhizoma coptidis; (◆) Gardenia; (●)Areca catechu.
Fig. 5. The influence of the concentration of rare earth on dye-up-take. (■) Caesalpinia sappan; (▲) Rhizoma coptidis; (◆)Gardenia; (●) Areca catechu.
Table 3. Influence of pH on the colorimetric data of the dyed fab-rics
DE L* a* b* C*
Caesalpinia sappan (pH=3) 49.89 61.64 03.93 56.98 57.11
Caesalpinia sappan (pH=7) 50.15 60.15 04.26 56.78 56.94
Caesalpinia sappan (pH=11) 51.68 61.38 03.89 55.89 52.03
Rhizoma coptidis (pH=3) 58.98 50.89 05.98 48.61 48.97
Rhizoma coptidis (pH=7) 60.65 50.48 07.03 46.18 46.71
Rhizoma coptidis (pH=11) 59.08 50.90 06.45 49.06 49.48
Gardenia (pH=3) 48.89 82.89 05.58 30.34 30.34
Gardenia (pH=7) 50.15 79.15 05.83 30.86 30.34
Gardenia (pH=11) 54.68 83.68 05.21 32.78 32.78
Areca catechu (pH=3) 38.58 68.08 10.93 41.78 43.19
Areca catechu (pH=7) 40.63 66.65 11.56 40.89 42.49
Areca catechu (pH=11) 39.28 66.98 12.89 40.23 42.24
Application of rare earth as mordant for the dyeing of ramie fabrics with natural dyes 2153
Korean J. Chem. Eng.(Vol. 28, No. 11)
on color shade, color measurements were performed on the fabrics
dyed with the four kinds of natural dyes. L* represents lightness
value in the CIELAB colorimetric system. Higher lightness value
means lower dye uptake [9,29,30]. a* and b* denote the red/green
value and the yellow/blue value, respectively [9,29,30]. The positive
values of a* and b* represent redder and yellower while negative
shows greener and bluer tones. C* stands for chroma or purity of
color [4,9,30]. ∆E refers to color difference [4,9,30]. As shown in
Table 3, the colorimetric data including ∆E, L*, a*, b*, and C* of
the dyed fabrics did not change much when pH of the bath changed
from 3 to 7 and 11. This is ascribed to the formation of stable com-
plexes among rare earth ions, natural dye molecules, ramie fibers.
The formation of the coordination bonds is irreversible. The result-
ant coordination compounds are highly stable in acidic, neutral and
alkaline baths. In addition, the multiple complexes formed by rare
earth ions and natural dyes can interact with each other, which can
result in the dissociation of the electron pairs on the both sides of
the complex conjugating system. The dissociated electrons can drift
among the complexes and thus generate the so-called ultrasensitive
transition effect, which changes the characteristic absorption spec-
trum. Thus the dyed fabrics can resist the pH fluctuation and main-
tain stable color shade in either acidic or alkaline bath. This is a sig-
nificant advantage of using rare earth as mordant in natural dyeing.
4. Color Fastness of the Dyed Fabrics
Table 4 shows the color fastness ratings to washing, rubbing and
light of the fabrics dyed with the four kinds of natural dyes in pres-
ence and in absence of rare earth as mordant. In comparison with
the fabrics colored via direct dyeing, the fabrics dyed having rare
earth as mordant exhibited much better color fastness to washing
and rubbing. The ratings of color fastness to washing and rubbing
of all the dyed fabrics having rare earth as mordant were found to
be grade 4-5. The fastness to washing and fastness to rubbing of the
fabrics dyed with the natural extracts depend on the type of mor-
dants, mordanting method and mordant concentrations as well as
molecular size and chemical structure of the dye, the dye-fiber or dye-
fiber-mordant interaction or bonds [7]. The most important factor
for determining the color fastness to washing and rubbing is the dye-
fiber-mordant interaction, which relies on the formation of stable
coordination bonds among rare earth ions and natural dyes and fibers.
The ratings of color fastness to light of the dyed fabrics using rare
earth as mordant were in the range 3-4 grade. However, the ratings
of fastness to light of the dyed fabrics without mordanting were found
to be grade 1-2. The color fastness to light of a natural dye is influ-
enced by the chemical structure of dye, physicochemical interac-
tion between natural dye and fiber, physical state of the dye inside
the fiber, dye concentration, chemical structure and physical char-
acteristics of the fiber itself, types of mordant, mordant concentra-
tions and mordanting method used [7,12,23]. Generally, it is well
known that the color fastness to light of natural dyes was poor [7,
23]. In this sense, the light fastness of dyed fabrics with rare earth
as mordant is considerably good. The formation of stable coordi-
nation bonds between rare earth ions and dye molecules and ramie
fibers accounts for the improvement of fastness to light of the dyed
fabrics.
5. Comparison of Different Metallic Mordants
As mentioned above, the lanthanum-rich rare earth chloride was
proved to greatly improve the color fastness, color strength and dye
uptake. Hence it is possible to replace the commonly used heavy
metallic salts with the rare earth chloride as the eco-friendly mor-
dant. The authors compared the rare earth mordants with the con-
ventional metallic mordants FeSO4 and K2Cr2O7. The efficiency of
the different mordants was explored by using RECl3·6H2O, NdCl3·
6H2O, FeSO4·7H2O and K2Cr2O7, respectively, as mordants in the
natural dyeing of ramie fabrics with Caesalpinia Sappan colorant.
Fig. 6 shows the influence of mordant concentration on dye uptake
and K/S values in presence of the different mordants. As expected,
with the increase of mordant concentration the dye uptake and color
strength increased dramatically at lower concentrations, suggesting
that all of the mordants can enhance dyeing effect significantly. After
a certain mordant concentration level, both the color strength and
dye uptake tend to a constant level for all the tests. Combining the
dyeing effect together with the consideration of using less metallic
mordant, the optimal concentration of RECl3·6H2O, NdCl3·6H2O,
FeSO4·7H2O and K2Cr2O7 was determined as 0.6 g/L, 0.6 g/L, 4.0
g/L and 1.6 g/L, respectively. As a result, the ionic concentration
employed for deriving optimal dyeing effect was [Nd3+]=0.24 mg/
L, [Fe2+]=0.81 mg/L and [Cr6+]=0.57 mg/L. The total rare earth
ionic concentration [RE3+] in the RECl3 solution should be close to
[Nd3+]. Apparently, employing rare earth mordent can greatly reduce
the ionic concentration employed in natural dyeing. The coordina-
tion number of rare earth elements is larger that that of the Fe2+ and
Cr6+. The coordination number of d-block transition metals is usu-
ally 4 or 6. In contrast, the most common coordination number of
rare earth ions is 8 or 9, because when a rare earth ion is coordi-
nated to ligands its 6s, 6p and 5d orbits usually participate in the
formation of coordination bonds [27,28]. In addition, the larger ionic
Table 4. Ratings of color fastness of the ramie fabrics dyed with rare earth as mordant
Washing Wet rubbing Dry rubbingLight
Fade Strain Fade Strain Fade Strain
Caesalpinia sappan (mordanted) 5 4 3-4 3-4 4-5 4 3-4
Caesalpinia sappan (direct) 3 2-3 2-3 2 3 3 2
Rhizoma coptidis (mordante) 4 4 3-4 4 4 4 3
Rhizoma coptidis (direct) 3 3 2 2 2-3 3 1-2
Gardenia (mordanted) 4-5 4-5 4 3-4 4 5 3-4
Gardenia (direct) 2 2-3 2 2 3 3 2
Areca catechu (mordanted) 4 4-5 4 3-4 4 4-5 3
Areca catechu (direct) 2 3 3 2 3 3 2
2154 G. H. Zheng et al.
November, 2011
radius of lanthanide elements accounts of the larger coordination
number of rare earth complex. Hence using rare earth chlorides as
mordants in natural dyeing can avoid heavy metal pollution. In add-
ition, the presence of metallic mordant usually changes the color
shade of dyed fabrics in natural dyeing. For example, Fe2+ usually
results in the darkening of the dyed fabrics. Using rare earth as mor-
dants can also enlarge the design flexibility in the dyeing process to
derive the desired color shade.
CONCLUSION
The rare earth was employed as mordant for the dyeing of ramie
fabrics with the natural extracts of caesalpinia sappan, rhizoma copti-
dis, gardenia and areca catechu. The effects of pre-mordanting, simul-
taneous mordanting, and post-mordanting on the dyeing effect were
compared. The dyeing temperature of 90oC was determined as the
best dyeing temperature, while the optimal dyeing time for the four
kinds of natural dyes was in the range 50-70 minutes. The highest
dye uptake was obtained when the dyeing bath pH was 7 or 8. The
fabrics dyed in presence of rare earth as mordant exhibited higher
color shade stability. On one hand, this is owing to the stable coordi-
nation bonds among the rare earth, natural dye and fiber. On the
other hand, it is ascribed to the interaction of the multiple complexes
formed by rare earth ions and natural dyes which enable the dyed
fabrics to resist pH impact maintaining the constant color shade. It
was proved that using rare earth as mordant apparently enhanced
the color fastness to washing, rubbing and light of the fabrics dyed
with the natural extracts. As compared with the mordants contain-
ing Fe2+ and Cr6+, the rare earth mordants can greatly reduce the ionic
concentration employed in natural dyeing. This study proved that
rare earth is promising in the application for mordant in the dyeing
with natural colorants.
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